As the density and complexity of microcircuits continue to increase, the photolithographic process used to print the circuit patterns becomes more and more challenging. Denser and more complex circuits require denser and more complex patterns consisting of smaller pattern elements packed more closely together. Such patterns push the resolution limits of available lithography tools and processes and place serious burdens on the design and quality of the photomasks used therein. To push the resolution limits, advanced photomasks are designed using various Resolution Enhancement Techniques (RET). Optical Proximity Correction (OPC) is one such technique. With OPC the photomask patterns are modified in various ways to help ensure that the printed pattern has good agreement to the original desired pattern. These photomask pattern modifications can include perturbations to the size of main pattern features, the addition of serifs to pattern corners, and the addition of Sub-Resolution Assist Features (SRAFs). None of these pattern perturbations are expected to survive the printing process. Instead they are expected to cancel pattern perturbations that would otherwise have occurred during the printing process. Although these OPC features help to preserve the fidelity of the printed pattern, they cause the photomask patterns to be even more complex than they would otherwise be. The increased complexity of the photomask pattern and fact that not all pattern elements are expected to directly effect the printed pattern makes the task of inspecting the photomask for meaningful pattern defects much more difficult.
In an effort to address the need for accurate photomask pattern inspection, many approaches have been applied with varying degrees of success. The most common methods of photomask inspection involve capturing high-resolution images of the mask pattern using either an optical or electron beam microscope and then comparing these images to reference images to look for defects. The reference images can be either acquired images of a second pattern on the photomask (die-to-die) or can be rendered from the design database (die-to-database). In either case differences between the images under test and the reference images are flagged as defects. Since these methods find defects by comparing high-resolution images of the photomask patterns they can be characterized as mask plane inspection techniques. Although these techniques are effective at finding mask defects they are susceptible to detecting high numbers of “nuisance” defects. Nuisance defects are real defects in the mask pattern that have little or no impact on the fidelity of the printed pattern. In the mask plane these nuisance defects may not be readily distinguished from other more serious defects. One measure of a defect's importance is its MEEF or Mask Error Enhancement Factor. This factor relates the size of the defect in the mask plane to the magnitude of the impact it will have on the printed image. High MEEF defects have high impact on the printed pattern; low MEEF defects have little or no impact on the printed pattern. An undersized main pattern feature in a dense fine-line portion of a pattern is an example of a defect with high MEEF where a small mask plane sizing error could cause a complete collapse of the printed pattern. An isolated small pinhole is an example of a defect with low MEEF where the defect itself is too small to print and is distant enough from the nearest main pattern edge so as not to affect how that edge is printed. As these examples show the MEEF of a defect is a somewhat complicated function of the defect type and the pattern context in which the defect is located. Without knowing the MEEF the mask plane inspection techniques must assume that all mask defects are important. At the same time these masks are so complex that they cannot be made free of all defects. Inspecting with enough sensitivity to find defects that may be important in high MEEF areas can lead to the detections of large numbers of similarly sized but unimportant defects in low MEEF areas. Time and energy can be wasted dispositioning these nuisance defects. Therefore, it would be advantageous to have a “MEEF aware” method that identifies lithographically significant defects while selectively screening out the nuisance defects in a timely manner.
Another method of photomask inspection that attempts to be MEEF aware involves optically imaging the mask pattern using a microscope whose illumination and imaging conditions mimic those of the wafer stepper. It is reasoned that to the extent that the microscope emulates the stepper, the defects will experience the same MEEF at inspection time as they will at time of use. However this approach suffers from many limitations that impair the effectiveness of this approach. One limitation is the limited applicability of this method to “in-process” inspection of photomasks. Some mask making sequences involve multiple process steps where the pattern is established in an early process step but the optical properties of the mask at that step are not those expected by the stepper. Because the unfinished mask does not behave like a finished mask in the inspection tool, the tool cannot properly take into account the MEEF of each defect. Also, this approach suffers from the limited flexibility with which the inspection microscope can be reconfigured. To precisely mimic a given stepper the microscope needs to precisely match the illumination and imaging conditions of that stepper. However, there are many stepper variations, each having a variety of possible configurations. Accordingly, it is difficult to build the required level of flexibility and precision into the inspection microscope of the inspection platform. Absent suitable emulation optics, the inspection tool cannot properly take into account the MEEF of each defect. Another limitation to the effectiveness of this approach concerns the difficulty in emulating the high-NA effects that occur at the wafer plane of a stepper. Known approaches sense the image at the emulated wafer plane using one or more image sensors. Practical sensors, however, have pixel sizes that are many tens of times larger than that that would be needed at the true wafer plane of a stepper. Correspondingly, the magnification of the inspection microscope must be tens of time higher than the magnification of the actual stepper emulated. However, with increased magnification comes substantially decreased NA. At a much lower NA, vector imaging effects and angle dependent resist film effects differ significantly from those that would be experienced at the true wafer plane. These differences limit the accuracy of the stepper emulation and again lead to poor accounting for the MEEF of the defects detected. Further limitations concern the general inadequacy of the photomask images acquired during inspection. Currently, these images are inadequate in terms of resolution, contrast and/or signal-to-noise ratio and thus are insufficient to enable adequate diagnosis of the nature of the defects on the mask. Therefore, an inspection method enabling the identification of photolithographically significant defects and being “MEEF aware” presents many advantages not present in conventional approaches known in the art.
Hybrid techniques for performing MEEF aware photomask inspections have been proposed. These techniques operate on a high-resolution image of the photomask to be inspected. From the image an estimated mask pattern is then input to a software simulation of the lithographic process that simulates the stepper and the resist yielding a simulated wafer plane image. Defect detection is then performed on a simulated wafer plane where the MEEF has already been taken into account. A serious shortcoming of these techniques involves the process by which a mask pattern is recovered from the high-resolution mask image. If the mask pattern (including defects) is known, than the high-resolution image of that pattern as seen by the inspection system optics can be determined by applying a proper partially coherent imaging model to the pattern. However, due to the highly non-linear aspects of such imaging models it is difficult to work backwards from the sensed image to the corresponding pattern. Heretofore, such techniques (those using approximations of this reverse transformation) have proven to be error prone and/or computationally expensive. Moreover, even the most computationally expensive approaches are subject to certain ambiguities and instabilities inherent in attempting to reverse a highly non-linear, lossy transformation. Although approximations can be made that are somewhat suitable over some range of inputs, all suffer from various limitations to their robustness that limit their applicability as part of a defect detection process.
Thus, although suitable for some purposes, each of the prior art techniques suffer from many limitations which substantially reduce their effectiveness in photomask inspection for meaningful pattern defects. Prior art processes are cumbersome, inaccurate, specialized, or inflexible and are not suitable from changing from one machine to another.
Accordingly, the embodiments of invention present substantial advances over the existing methodologies and overcome many limitations of the existing inspection arts. These and other inventive aspects of the invention will be discussed herein below.